An electro-optic material has at least two “display states,” the states differing in at least one optical property. An electro-optic material may be changed from one state to another by applying an electric field across the material. The optical property may or may not be perceptible to the human eye, and may include optical transmission, reflectance, or luminescence. For example, the optical property may be a perceptible color or shade of gray.
Electro-optic displays include the rotating bichromal member, electrochromic medium, electro-wetting, and particle-based electrophoretic types. Electrophoretic display (“EPD”) devices, sometimes referred to as “electronic paper” devices, may employ one of several different types of electro-optic technologies. Particle-based electrophoretic media include a fluid, which may be either a liquid, or a gaseous fluid. Various types of particle-based EPD devices include those using encapsulated electrophoretic, polymer-dispersed electrophoretic, and microcellular media. Another electro-optic display type similar to EPDs is the dielectrophoretic display.
Generally, an image is formed on an electro-optic display device by individually controlling the display states of a large number of small individual picture elements (“display pixels”). The one or more bits of data defining a particular display state of a display pixel may be referred to as a “data pixel.” An image is defined by data pixels and may be referred to as a “frame.” Commonly, the display pixels are arranged in rows and columns forming a matrix (“display matrix”). An exemplary electro-optic display pixel includes a layer of electro-optic material situated between a common electrode and a pixel electrode. One of the electrodes, typically the common electrode, may be transparent. The common and pixel electrodes together form a parallel plate capacitor at each display pixel, and when a potential difference exists between the electrodes, the electro-optic material situated in between the electrodes experiences the resulting electric field.
An electro-optic display may be of either the active or passive-matrix types. With active-matrix electro-optic displays, any particular display pixel in the display matrix may be addressed by driving a select signal on the row select line and simultaneously driving an optical-property-dependent signal on the column data line. However, in order to change the display state of a display pixel, particular types of display devices require driving the pixel electrode over time with a series of voltage pulses regularly spaced in time, i.e., the display pixels are driven with a waveform. The addressing of a particular display pixel in these display devices must be made in accordance with the timing requirements of the waveform used to change the display state of a display pixel. Accordingly, the use of an active-matrix electro-optic display device having display pixels driven with a waveform requires that the active-matrix addressing features be used in conformity with waveform timing requirements.
An electro-optic display device may have display pixels that have multiple stable display states. Display devices in this category are capable of displaying (a) multiple display states, and (b) the display states are considered stable. With respect to (a), display devices having multiple stable display states include electro-optic displays that may be referred to in the art as “bistable.” The display pixels of a bistable display have first and second stable display states. The first and second display states differ in at least one optical property, such as a perceptible color or shade of gray. For example, in the first display state, the display pixel may appear black and in the second display state, the display pixel may appear white. In addition, display devices having multiple stable display states include devices having display pixels that have more than two stable display states. Each of the multiple display states differ in at least one optical property, e.g., light, medium, and dark shades of a particular color. As another example, a display device having multiple stable states may have display pixels having display states corresponding with 4, 8, 16, 32, or 64 different shades of gray.
With respect to (b), the multiple display states of a display device may be considered to be stable, according to one definition, if the persistence of the display state with respect to display pixel drive time is sufficiently large. The display state of a display pixel may be changed by driving a voltage on the display pixel until the desired appearance is obtained. Alternatively, the display state of a display pixel may be changed by driving a series of voltage pulses regularly spaced in time. In either case, the display pixel exhibits a new display state at the conclusion of the drive time. If the new display state persists for at least several times the minimum duration of the drive time, the new display state may be considered stable. Generally, in the art, the display states of display pixels of LCDs and CRTs are not considered to be stable.
An important advantage of electro-optic displays having multiple stable display states, in general, and EPD devices, in particular, is that once a display pixel has been placed in a particular display state, the display pixel will maintain that display state for a long period of time—at a minimum one or more minutes and up to hours, days, months, or longer—without drawing power. EPD devices need only be refreshed when a change in the appearance of the rendered image is desired or after the brightness of the rendered image diminishes below a desired level. In contrast, other types of display technologies maintain their display state for much shorter time periods. For example, the display pixels of a liquid crystal display (“LCD”) maintain their optical appearance for less than a second. However, in comparison with other display technologies, such as LCDs, EPD devices require relatively long drive times to cause a display pixel to assume a new display state. Thus, changing an image rendered on an EPD device may take longer than desired.
Accordingly, there is a need for efficient methods and apparatus for updating an electro-optic display device having display pixels having multiple stable display states, each display pixel requiring a series of voltage pulses regularly spaced in time to change its display state.